In current optical communication systems, signals are transmitted long distance using multiple wavelengths of light passing through optical fibers. Each optical carrier wavelength (or channel) can be encoded with a unique set of information. The broader the optical bandwidth of the transmission system, the more information can be transmitted using more wavelength-division multiplexed (WDM) signals. Such WDM optical systems use optical fibers, which produce some level of optical loss, typically in the range 0.15-0.3 dB/km. Additionally, components used in these systems to perform signal enhancement or signal processing functions such as dispersion compensation or dynamic equalization, add optical loss. In order to overcome these losses and maintain the optical signal to noise ratio (OSNR) of each channel, optical amplification is required periodically. Such optical amplification must be broadband, at least as broadband as the wavelength range of signals to be transmitted and its gain must be close to constant for all signal wavelengths (gain flat) so that all signals experience nearly the same gain. Additionally, the amplification must not add much noise to the amplified signal, as represented by a low amplifier noise figure (NF).
Unfortunately, the gain of most optical gain media is not flat across a wide range of optical wavelengths. However, gain flatness can be achieved using an optical gain flattening filter (GFF), which is a device that creates a predetermined wavelength-dependent optical loss to perfectly compensate for any gain flatness error. Such a filter is typically placed within an amplifier to achieve gain flatness to some tolerance level.
A characteristic of optical fiber is its optical chromatic dispersion, which is a measure of the difference in propagation speeds of different wavelengths of light in the fibers. Too much dispersion leads to a spreading of pulses and a degradation of the optical signal to noise ratio (OSNR). Too little dispersion can lead to degradation caused by nonlinear optical effects. To tailor the dispersion for optimal performance, systems are often designed containing devices that compensate for dispersion, so that all wavelengths contained in a signal arrive at the receiver at the same time. These dispersion-compensating modules (DCMs) create optical loss and are often added within the system inside the optical amplifier or between stages of amplification, a design decision that is known to advantageously minimize the accumulation of optical noise.
While optical gain is possible in many different gain media, in most currently deployed optically amplified communication systems, the gain medium consists of erbium ions doped into a silica-based fiber. Such Erbium-doped Fiber Amplifiers (EDFAs), when provided with sufficient optical pump radiation from available pump diodes, can provide efficient low noise amplification at the low loss window of optical transmission fibers, namely near 1550 nm. EDFAs can produce gain across a 40 nm window from 1525-1565 nm (called the C-band) or can be designed differently to produce gain from 1565-1605 nm (called the L-band). In both bands, the gain is not adequately flat for most WDM optical communications systems without the inclusion of some filtering, and the shape of the gain varies with operating condition.
As part of the expansion of the information carrying capacity of optical networks, it is necessary to install additional equipment to add capacity to the amplifiers (optical repeaters) in the optical transmission path. This generally will require additional wavelength carrier channels, possibly operating at higher data rates, which need to be amplified within the network optical path. In addition this may require the addition of signal conditioning components such as gain flattening and dispersion compensation to properly handle the larger bandwidth of the network.
Current methods to expand channel capacity in a currently installed system are: 1. New Amplifier Design: Design a new amplifier to handle the higher channel count (and data rates) to replace the lower channel count amplifiers in the network, or 2. Parallel Expansion: Break out the channels into parallel banded regions of the optical spectrum (consisting of 1 to as many as 40 or more channels) Each band can then be amplified by a separate amplifier which is gain flat in a specific narrow band and then the bands can be recombined for transmission in the network. The new design approach is expensive and is customized to each expansion of the channel capacity. Parallel expansion of the channel count means that banded groups of channels must be broken out (de-multiplex) from the transmission path and then each banded group requires a unique amplifier. For example, sub-bands each consisting of 4 channels, and 40 total channels used in the network this means that 10 separate amplifiers are required for a fully utilized network. Such an example of using the parallel band expansion of system channel capacity is shown in FIG. 1. The signals in the network transmission fiber must be demultiplexed into N sub-band groups. Each sub-band group is then amplified independently. As additional capacity is required in the network more sub-band paths are added to the system. Spectral guard bands are generally necessary to avoid interference between sub-bands which means that some of the bandwidth capability of the EDFA's is not used in this approach. The channels must then be recombined (multiplexed) back into the transmission fiber path.
In general the up front costs are high for the banded parallel expansion strategy since the channel breakout for the anticipated capacity of the system is almost always installed when the system is built. Building additional breakout capacity as needed could be even more expensive and add loss (a detriment to NF performance) to the channel paths. In addition the need to design amplifiers for each banded set of channels adds more complexity and cost to the system.
Signal conditioning elements such as DCMs or GFFs are not shown in the example of the parallel approach in FIG. 1. Signal conditioning could be done at the sub-band level, which require that N DCMs and GFFs for a fully utilized system. Signal conditioning that needs to be performed on the transmission fiber level can require an additional amplification stage, adding more cost to the system.
Accordingly, it would be desirable to provide an optical amplifier arrangement that can be expanded to handle additional network capacity that is simpler and less expensive than current approaches.